The Wright Bros wanted an engine that developed 8 horsepower and weighed less than 200 pounds. They quickly discovered that no one made such an engine in 1903. They would have to make their own, from scratch. They cast their own engine block in aluminum. According to later reminiscences by Charles Taylor, the Wright's "mechanician" who built the engine, there were no drawings, it was all built from sketches. Contrary to modern practice, the pistons were cast iron rather than aluminum. It had no carburetor, and no throttle. Gasoline ran out of a tube into the intake manifold where engine heat vaporized it. Lacking a throttle, the engine ran at full power all the time. It had a chain driven camshaft that worked the exhaust valves and the ignition points. Intake valves were held closed with springs, and opened from the suction of the piston going down, no cam drive as is normal today. The hand built engine exceeded spec, producing 12 horsepower and only weighing 180 pounds.
All this from this week's Aviation Week, which is celebrating their 100th anniversary.